Complex landslides in clay-bearing sediments are investigated in two moderate-relief regions of Southeast Spain. Both regions, more than 100 km apart show landslides affecting the same Flysch formation, which outcrops widely in the central and western Betic Cordillera along the contact between the External (South Iberian Domain) and Internal (Alborán Domain) zones. Intense rainfall episodes can be considered as the main triggering factor for slope failures in these two areas. We have chosen two landslides (Diezma and Riogordo landslides), one from each area of study, to investigate their morphological and geotechnical features in order to establish the relative importance of the different controlling factors. From a kinematic point of view, the two features studied in detail can be referred as to rotational failures, evolving downhill to slow earthflows. The movement was concentrated on several surfaces developed on a clay-rich layer mostly constituted by smectite. This clay mineral is of critical relevance to the mechanical behaviour of soils and Flysch-like formations, being very consistent at dry conditions, but rapidly losing its strength at wet conditions. Thus, softened smectite-rich clay layers with high water contents can approach the properties of a lubricant, which, in turn, can be critical for slope stability. In addition to their high plasticity, these clays have a high swelling potential, which can induce significant vertical overpressure, thus reducing even more the strength properties of the Flysch formation. In Southeast Spain, a region with a Mediterranean rainfall regime, slope stability can be seriously influenced by the presence of these smectite-rich clay layers in a formation of regional extent, as is the case of the Flysch formation. Therefore, this lithologically-controlled factor should be taken into account when evaluating landslide hazard in the Betic Cordillera.

A complete failure analysis of a complex landslide located in Southern Spain (the Diezma landslide) has been undertaken using geotechnical, geophysical and geological data. The triggering factors were a high groundwater level and the reduction in the shear strength parameters of the high-plasticity clay. The 2010 reactivation of the landslide was related to the poor performance of the first line of deep drainage wells, although the second and third line of wells and the anchored pile barrier were effective in preventing the landslide reaching the A-92 motorway. Analysis indicates that further reactivation of the Diezma landslide could take place following a period of heavy rain if the drainage wells are not properly maintained, or if an earthquake of M w 4.0-5.0 occurs within 25 km of the site.